The invention relates to means for cooling the faceplate of the cathode ray tube in an in-line projection system for a television receiver. Such a system (hereinafter referred to as ("a television projection system of the kind described") comprises a transmission lens arranged in front of and in line with a cathode ray tube.
It is known in such projection systems to use a liquid to dissipate the heat generated by the very bright, high-energy cathode ray tubes employed in these systems. The liquid, interposed between the transmission lens and the CRT (cathode ray tube) faceplate, also serves to improve picture brightness by eliminating the glass-to-air optical interfaces formed at the rear surfaces of the lens and the front surface of the CRT faceplate. It also avoids the need to work these surfaces to optical flatness. In one known arrangement, described in British Patent Specification No. 417,435, liquid is contained in a space which is bounded on two opposite sides by the lens and the CRT faceplate, respectively, and around the periphery by a rigid collar having an inlet and an outlet for circulation of the liquid for cooling purposes. With this arrangement, due to the presence of the rigid collar, which is fixed to the lens and to the cathode ray tube, it is not possible to adjust the whole lens axially relative to the tube to focus the projected image onto the screen of the television receiver. To permit such adjustment it has been proposed to use a corrugated flexible bellows in place of the rigid collar. This does not, however, obviate a further disadvantage of the known arrangement, which is that, in cases where the lens is to be made of a plastics material, the lens maker is limited to a choice of such materials which are compatible with the cooling liquid. This drawback is avoided in another proposed arrangement in which a chamber for containing cooling liquid is formed between the CRT faceplate and a glass plate which is spaced from the front of the faceplate and is connected to the faceplate by a peripheral wall. This arrangement, however, leaves glass-to-air interfaces at the front of the glass plate and the rear of the lens, resulting in loss of light and, therefore, picture brightness due to surface reflection.
A further drawback common to all these arrangements is that the space for containing the cooling liquid, since it is partially bounded by the CRT faceplate or by the faceplate and the lens, has to be formed during the manufacture of the cathode ray tube or during the assembly of the tube and the lens, with the result that these activities become more complicated and require the provision of additional on-site skills and facilites. It is an object of the invention to provide a cooling means which does not suffer from this drawback.